Internal combustion engines having two separate banks of cylinders are known, where in each case an exhaust gas turbocharger is assigned to each cylinder bank. The two exhaust gas turbochargers consist in each case of an exhaust turbine located in the exhaust section of the two cylinder banks and a compressor which is located in the intake section of the two cylinder banks and increases the charging pressure.
In the event of a blockage affecting a bi-turbocharger of such a type the drive power converted at the compressor in question is reduced. The reason for this is the fact that the blockage results in a reduction of the volume flow, as a result of which the charging pressure of the exhaust gas turbocharger in question decreases. In the case of such a single-sided drop in power affecting the bi-turbocharger, the other exhaust gas turbocharger will deliver a correspondingly greater volume flow in this situation, which can lead to mechanical overloading as a result of the increased rotational speed of the exhaust gas turbocharger which is functioning correctly. It is therefore important in the case of a bi-turbocharger that deviations in rotational speed between the two exhaust gas turbochargers due to malfunctions are detected in good time in order to prevent damage to the fault-free exhaust gas turbocharger caused by excessive rotational speeds.
A method is known from EP 0 856 097 B1 for detecting differences in rotational speed between the two exhaust gas turbochargers of a bi-turbocharger. To this end, a pressure sensor is located in the exhaust section of each of the two cylinder banks, which measures the exhaust pressure. It is then possible to infer the difference in rotational speed of the two exhaust gas turbochargers from the difference between the two exhaust pressures. In this situation, it is assumed that the difference in pressure between the two cylinder banks of the internal combustion engine increases with the difference in rotational speed. If the difference in rotational speed between the two exhaust gas turbochargers exceeds a predefined maximum value over a specific period of time, protective measures are initiated which are intended to prevent any damage occurring. For example, in such an instance a bypass valve (waste gate) can be opened which bypasses the exhaust gas turbine of the exhaust gas turbocharger which is functioning correctly and thereby reduces its power.
The disadvantage of the known method described above for monitoring the rotational speed of a bi-turbocharger is the fact that a pressure sensor is required in the exhaust section of each of the two cylinder banks.